In lung cancer, bombesin/gastrin releasing peptide (BB/GRP) is an autocrine growth factor. Here the effects of a new BB analogue (Tyr0,Bpa4)BB were investigated using small cell lung cancer (SCLC) cells. (Tyr0,Bpa4)BB bound with high affinity to SCLC cell line NCI-H345 (Kd = 2 nM). Upon exposure to ultraviolet light, bound (Tyr0,Bpa4)BB covently labeled a 75 Kdalton protein, the GRP receptor. (Tyr0,Bpa4)BBinternalized at 37oC but not 4oC. After addition to NCI-H345 cells, (Tyr0,Bpa4)BB caused elevation of cytosolic Ca2+ and c-fos mRNA. These results suggest that (Tyr0,Bpa4)BB is a GRP receptor agonist.BW2258U89 is a gastrin releasing peptide (GRP) receptor antagonist which inhibits SCLC growth. BW2258U89 was metabolized by mouse plasma with a half life of 170 minutes. Using mass spectroscopy techniques, the BW2258U89 metabolites had a molecular weight increase of 1 dalton suggesting that BW2258U89 was deamidated (Da). (Da)BW2258U89 inhibited (125I-Tyr4)BB binding to NCI-H345 SCLC cells with an IC50 value of 450 nM whereas BW2258U89 bound with 25-fold greater affinity. BW2258U89 antagonized the ability of BB to elevate cytosolic Ca2+ and increase c-fos mRNA in NCI-H345 cells, whereas (Da)BW2258U89 did not. BW2258U89 inhibited NCI-H345 clonal growth whereas (Da)BW2258U89 did not. These data suggest that an amidated C-terminal is important for BW2258U89 to antagonize GRP receptors.